Printed circuit boards (PCBs) comprise one or more layers of conductors, typically copper, separated by insulting layers such as glass, epoxy, or polyimide on which electronic components are physically mounted, providing mechanical support for electronic circuitry. By soldering components' leads onto the PCB's conductive traces, electronic devices such as integrated circuits, transistors, diodes, resistors, capacitors, inductors, and transformers are electrically interconnected to form electronic circuits. Applications of PCBs include virtually every type of electronic product including cell phones, cameras, lithium ion batteries, tablet computers, notebooks, desktops, servers, network equipment, radios, consumer devices, televisions, set top boxes, industrial electronics, automotive electronics, avionics, and more. FIG. 1 illustrates various examples of printed circuit boards reflecting their diversity in fit, form, and function. In medical, sports, and select consumer electronic devices, PCBs may also be employed in “wearable” electronics, devices that are required to conform to the curved surfaces of the human body.
In electronics, the roles of a PCB are two-fold, firstly mechanical, by functioning as a passive substrate to provide support for electronic components mounted either on the top or alternatively on both the top and bottom of the PCB, and secondly electrical, providing multi-layer interconnections between these components and electrical connectors. In contrast to integrated circuits, where the silicon substrate functions both as mechanical support and the material used to fabricate and form active integrated semiconductor devices, a PCB substrate is “passive” acting only as an insulator. The insulating PCB substrate, also known as a base laminate, may be rigid, flexible, or rigid-flex, as shown in FIG. 2. Rigid PCB 10 comprises an inflexible substrate to which all components and connectors are attached. In contrast flex PCB 11 comprises a flexible circuit board to which components and connectors are attached. Rigid-flex PCBs combine both rigid PCB portion 12 and flex PCB portion 13 combined together into one PCB. Components and connectors may be mounted on either the rigid or flex portions as needed. Each type of PCB offers specific advantages and disadvantages as described in the following sections. A general overview of rigid, flexible, and rigid-flex PCBs is discussed online at https://en.wikipedia.org/wiki/flexible_electronics.
Rigid PCBs
A rigid PCB is one that does not bend, deform, or flex significantly when subjected to mechanical stress. Rigid PCB technology is by far the most popular PCB technology used today, common for any flat or encased product including cell phones, tablets, computers, TVs, and even kitchen appliances. One advantage of a rigid PCB is the substrate absorbs mechanical stress thereby suppressing damage to components and their solder joints. One disadvantage of rigid PCBs is they are intrinsically planar and cannot bend to fit curved surfaces. As such they are not considered good a good solution for bendable or wearable applications. (Note: As used herein, the term “rigid” is not used in an absolute sense, but rather to mean that the object in question (typically a PCB) does not bend significantly or permanently when exposed to bending forces and will return to its original shape when the bending forces are removed. In particular, the term “rigid,” as applied to a PCB, is used in a relative sense to mean that the PCB is more rigid than a flexible PCB to which the rigid PCB is connected.)
Rigid PCB substrates typically comprise phenolic, polyimide, plastic, or other stiff non-conductive materials. One common material used in rigid PCB manufacturing is FR4, an acronym for “fire retardant” material, comprising a woven fiberglass cloth pre-impregnated with epoxy resin. Such substrates may also be referred to as “prepreg” sheets, an abbreviation for preimpregnated bonding sheet. In the manufacturing process known as “lamination”, sheets of copper foil are coated, i.e. “laminated” onto the prepreg sheets. During fabrication the combination of pressure and heat activates epoxy resin in the prepreg sheet, causing it to flow conformally between the foil and prepreg sheets, bonding them together. In this context, the term laminate means to unite layers of materials by adhesion or other means into a flat sheet or sandwich, which may be rigid or flexible. The process can be repeated multiple times to create multilayer PCBs. A more detailed description of the well known laminated PCB manufacturing process is described online in the document http://www.4pcb.com/media/presentation-how-to-build-pcb.pdf.
For performing electrical interconnection, rigid PCBs range from single layer PCBs, having only one conductive layer, to multilayer sandwiches comprising four, six or even ten conductive layers of copper “foil” needed for realizing complex systems. In “single layer” PCBs, the copper layer is laminated or plated on only one side of the insulating substrate, with all the components mounted on the same side of the PCB. In “dual-layer” PCBs, the same base insulating laminate is clad with copper on both sides and electronic components may be mounted on either or both surfaces of the PCB. Multi-layer PCBs comprise more than two layers of copper foil clad onto intervening layers of insulating material to form the multi-layer sandwich. The number of layers refers to the number of conductive copper layers in the PCB, e.g. a “four-layer” PCB has four copper layers with three intervening insulating layers together comprising a laminated sandwich of seven layers. The outer copper layers may also be coated with a protective layer for protection against scratches and corrosion, but such protective layers are not considered as part of the lamination process.
Depending on its intended application, copper thickness varies with the amount of copper needed to form each conductive layer in a PCB. Rather than describe each layer by its precise layer thicknesses, for convenience's sake the PCB industry typically describes laminate copper thickness in terms of its “weight”, where the layer thickness is linearly proportional to this weight. For historical reasons, PCB industry vernacular refers to copper weight in English units of “ounces” as measured on an area of one square foot. For example, a PCB with 0.5 oz. copper has a copper thickness of 0.7 mils or 17.5 μm; a PCB with 1.0 oz. copper has a metal thickness of 1.4 mils or 35.0 μm, 2 oz. copper has a metal thickness of 2.8 mils or 70 μm, and so on.
Extreme copper thicknesses resulting from 20 oz. to 30 oz. copper can be used for high currents and in power electronics. Thick copper becomes extremely rigid and incurs high stress between the copper and the PCB resulting from differences in the TCE, i.e. the temperature coefficient of expansion, of the dissimilar materials. Extreme stress can lead to a variety of failure modes in a PCB, including board cracking, delamination of the conductive layers, and solder joint cracking.
In PCB manufacturing, copper layers are patterned to form electrical circuits generally through the process of “photolithography”. The patterning is performed on a layer-by-layer basis starting with a uniform un-patterned copper laminate clad across the entire planar surface of the insulating substrate. In photolithography, the copper layer to be patterned is first coated with a light sensitive emulsion known as a “photoresist” typically applied in sheets of “dry-film” using heat and pressure. To transfer an image to the resist, an optical mask or “photomask” is used to control which portions of the dry resist sheet are exposed to light and which are not. The photomask is first created using commercially available CAD software resulting in a “gerber file” defining the mask pattern needed for mask manufacturing. The resulting photomask may contain features at the same size as those to be defined on the PCB, or may be optically scaled up or down using an optical instrument known as a “mask aligner” used to align the projected photomask image to any other features already present on the PCB.
Next the photoresist is exposed to light through the patterned mask thereby transferring the image. The photoresist is sensitive to exposure to short wavelength light such a ultraviolet light, but not to longer wavelength visible light, e.g. colors such as yellow or red light. After exposing the photoresist, the resist is “developed” causing the photoresist to be washed away in some regions and retained in others as defined by the portions of the photoresist exposed to light and those is the shadow of the photomask. After developing the photoresist, organic photoresist layer mimics the pattern of the mask through which it was exposed, covering the copper metal in some regions and not in others.
The metal portions that are protected by the photoresist and those that are exposed to etching depend on whether a “positive” or a “negative” photoresist is employed. Positive and negative photoresists react to light in an opposite or complementary manner. Specifically, for positive photoresist, any photoresist regions exposed to light causes the exposed chemical bonds to break, washing away that portion of the photoresist during the developing process. Since photoresist is removed in the light exposed areas, then only in the shadow of the photomask features is photoresist retained, meaning that the remaining photoresist pattern exactly duplicates the photomask features, i.e. dark areas are protected from etching. Everywhere else the metal will be etched away.
In the case of negative photoresist, any photoresist regions exposed to light causes the exposed chemical bonds to cross-link, not break, preserving only the exposed portions of the photoresist during the developing process and washing away the photoresist in the photomask's shadow. Since photoresist is preserved only in the light exposed areas, all dark areas in the mask will be result in unprotected metal to be etched away. The resulting PCB features are therefore exactly opposite, i.e. the negative image, of the photomask.
So the mask polarity, i.e. the dark features and clear portions of the photomask, must correspond to whatever photoresist is employed in the masking operation. After exposure, the photoresist is “hard baked” at a high temperature to strengthen it to withstand prolonged exposure to acid etches. Because the photoresist comprises an organic compound, it is relatively insensitive to exposure to acids, especially after hard baking. The metal is then etched in acid and thereafter the mask is removed. Copper etches generally employ nitric, sulfuric, or hydrofluoric acids either in pure form, diluted by water, or mixed either hydrogen peroxide or some other compound. Ferric chloride or ammonium hydroxide may also be used. The composition of various copper etches can be found online, for example at http://www.cleanroom.byu.edu/wet_etch.phtml.
The photolithographic process must be repeated for each copper layer used. For example, in two-sided PCBs, copper interconnects are laminated on both sides of the intervening insulator and using photolithography, each side must be patterned separately using different masks unique the specific circuit layer. Interconnections of the two sides through the insulating layer are facilitated by conductive vias. A conductive via is a mechanically drilled hole lined or filled with a conductor metal such as a plated metal. The concept of a two layer PCB can be extended to 3, 4, 6 or 8 layer PCBs simply by repeating the processes of lamination, photolithographic patterning, and via formation. Conductive vias may interconnect any two conductive layers, or reach entirely through every layer of the PCB.
Although an entire electronic system can be integrated onto a single rigid PCB, in many instances, the resulting PCB is too large or has the wrong shape to fit in available space. In such cases, the system must be broken into two or more PCBs employing wires or cables between PCBs to facilitate electrical interconnection of the various constituent PCBs. For example, FIG. 3 illustrates an application requiring numerous rigid-PCBs 21 housed in a flexible polymeric pad 22 to form device 20, an LED light-pad used in medical phototherapy applications and designed to bend in one direction in order to conform to various body shapes, e.g. an arm, leg, etc.
As shown, rigid PCBs 21 are interconnected to one another through ribbon cables 27 and associated ribbon cable connectors 28. Using plug and socket type ribbon cable connectors, ideally the inter-board connections electrically behave the same as an on-board connection between two components mounted and connected on a common PCB. In reality, however, the wiring from board-to-board introduces parasitic resistance, capacitance, and inductance that can distort sensitive analog signals, interfere with radio frequency (RF) communication, emit electromagnetic interference (EMI), and limit data communication and clock rates to low frequency operation. These parasitic elements also can adversely impact power distribution and affect voltage regulation accuracy or stability. Moreover, because the flexible pads are positioned in various locations across a patient's body, normal application of the product repeatedly subjects the cable to movement, twisting and pulling.
Repeated movement puts mechanical stress on the solder joint between the wire and the PCB trace, eventually leading to a broken wire or a cracked solder joint, for example on the solder joints connecting discrete wires 24 onto rigid PCB 21 and needed to connect electrically connect rigid PCB 21 to cable 23. In order to reduce stress on the solder joints between the wires and the PCB, strain relief 26 and added support 25 have been included to prevent damage from wire pull during use of device 20. Despite these precautions, electrical connections subjected to repeated flexing, bending, and wire-pull exhibit poor long-term survivability and suffer frequent reliability failures.
Example of PCB interconnection failures include frayed wires 35 and broken wire 36 shown in FIG. 4.
Replacing discrete wires with plugs and connectors can reduce the incidence rate of solder joint failures but introduces several new failure modes including wires being pulled from the plugs in the connector failures 53 and 54 shown in FIG. 5. An alternative interconnection method to eliminate the use of separate wires employs the use of multi-conductor ribbon cables terminated by plug and socket connections
In such solutions, sockets are soldered directly onto the PCB and plugs are mechanically and electrically connected to the ribbon cable. To carry the required current, more than one conductor may be required for power connections such as ground or +V (power). In manufacturing, the connector socket is attached onto the PCB at the same time as other components, typically using surface mount technology (SMT) production lines to solder all the components onto the PCB at one time. Attaching the plug to the ribbon cable does not normally utilize solder but instead employs a mechanical technique forcing metal blades to penetrate the ribbon cable's wire insulation connecting each wire in the cable to its own dedicated pin in the plug. During final assembly the plug is pushed into the socket completing the connection.
In applications with repeated movement and flexing, plug and socket connections suffer several failure modes—the most common failure comprising a case where the plug comes loose from the socket and no longer makes a reliable connection between the plug pins and the socket's conductors. Utilizing a clamping socket—a socket that uses tension or a spring-loaded clip to hold the plug securely in place, can largely circumvent socket disconnection failures. Unfortunately, clamping sockets eliminate one failure mode but introduce a new failure mode in the cable. Specifically, if the plug is held tightly in place, during movement, twisting, or pulling, the connection between the ribbon cable and the plug will fail.
Regardless of whether repeated movement or flexing results in an unplugged connector or a broken cable, the interconnection between PCBs will fail and an open circuit will result. In systems comprising a large number of rigid PCBs, e.g. in a series of PCB's used to cover a large area, the number of interconnections further exacerbates the problem with each connector statistically increasing the probability of system failure.
While the use of ribbon cable and their associated plugs and connectors reduce the risk of system failure from wired connection failure-modes such as wire pull or solder joint cracking, ribbon cable is still subject to single point system failure, i.e. where a single wire break results in partial or total system malfunction. For example, if a control wire is broken, the system will not be able to receive commands. In cases where two wires are required to carry the required current, breakage of either wire will cause a single wire to carry too much current leading to excessive voltage drops, overheating, instantaneous wire fusing, or electromigration failure over time.
Insuring PCB connection reliability is especially problematic in applications subject to repeated cycles of flexing. For example, in bendable polymeric pads 73 used in medical phototherapy such as shown in FIG. 6, an integrated circuit comprises a PCB 70 with integrated circuit 71 and LEDs 72 where the components are housed in rigid plastic packages. During phototherapeutic treatment, infrared and (select wavelengths of) visible light 75 from LEDs 72 traverses transparent sanitary barrier 77 penetrating into tissue 76. To insure consistent penetration depth into tissue 76, polymeric pad 73 and flex PCB 70 must bend to match the shape of the body part being treated.
Each flexible polymeric pad is part of a larger system comprising a set of three pads 80a, 80b, and 80c shown in FIG. 7A. Pad 80a connects to an electronic driver circuit (not shown) through plug 81 and cable 82 with strain relief and cable connection 83 and to pads 80b and 80c through connector cables 85a and 85b and socket 84. The pads are attached to Velcro straps 88 glued in place and bent into shape by pressure from Velcro belt 87. FIG. 7B illustrates the resultant bending in actual use treating knee and leg 91 in medical application 90 and when treating leg 96 in equine veterinarian application 95. In such cases, the flexible polymeric pads 80a, 80b and 80c and their components therein, along with Velcro straps 88, all undergo significant bending stresses and deformation during treatment with repeated flexing cycles each time the pads are reapplied to new patients or treatment areas.
In the event that rigid PCBs are employed, damage to PCBs from deformation as shown in FIG. 8A may include cracked PCB coating 101 in PCB 100 or cracked substrate 103 and broken traces 104 in PCB 102. Another failure mode is cracking of the conductive vias 105 as shown in FIG. 8B. Despite the small size of horizontal hairline crack 106, via 105 is an open circuit. To avoid rigid PCB breakage, a flex PCB can be used for realizing flex circuits as described next.
Flexible PCBs
An alternative solution to implementing a system comprising an array of interconnected rigid PCBs is to utilize a flexible PCB such as shown in FIG. 9. In contrast to rigid PCBs, a flexible PCB is one that bends, flexes, or twists with torque. Flexible PCBs bend on three axes, providing either two-dimensional or full three-dimensional movement depending on their application. Flexible PCBs are often used as a replacement to ribbon cable connectors or to replace rigid PCBs in restricted spaces and tightly assembled electronic devices. Applications employing flex PCBs as interconnects included ink jet printers, flip type cell phones, computer keyboards, and other moving apparatus such as the moving arm in hard disk drive data storage.
Most flex PCBs comprise only passive circuits for interconnection. In some instances flex PCBs may also include components mounted on one or both sides of a flex PCB primarily for fitting into small enclosures such as automotive, industrial and medical device modules. Flex PCBs with attached components are also referred to as flex circuits. Flex PCBs generally utilize much thinner copper layers and thinner insulating substrates than rigid PCBs. Substrates may involve polyester, silk, polyimide, semi-crystalline thermoplastics (also known as PEEK polymers), or flexible plastics and polymeric materials. Like rigid PCBs, flex PCBs may comprise, single, dual or multi-layer constructions generally with conductive vias.
The construction of a flex PCB depends on its intended use. Flex PCBs operating purely as “flex connectors” typically comprise one to four layers and do not contain any components mounted on either side of the flex PCB's surface. In use, such flex-based connectors may be flexed “frequently”, i.e. alternating between a flexed (bent) and un-flexed (straight) condition over and over again at regular intervals; flexed “occasionally” seldom changing between flexed and un-flexed states, and flexed “rarely” meaning the shape of the PCB is bent into position during manufacturing and remains unchanged thereafter. In the context of this application, the term “flexing, does not mean simply being in a bent state, but in the metaphor of weightlifting means alternating between being in a straight and bent state repeatedly, generally in repeated cycles.
One common example of a flexed-frequently application includes the flex connector attached to a printer head in an ink jet printer. A flex-occasionally application includes the flex connector connecting a notebook computer's display housed in a hinged lid to the main body of the computer containing its keyboard and motherboard PCB. In this example, each flexing cycle repeats occasionally, i.e. each time the notebook computer is opened and then closed again.
In contrast, flex-infrequently applications of flex PCBs, either for realizing flex PCB connectors or for flex circuits, are best suited for their ability to fit into small, curved, or oddly shaped enclosures as part of the manufacturing process, and are not intended to be used in applications with repeated flexing cycles. Applications of rarely flexed PCBs include a flex connector in a bar type cell phone or a digital camera, where the flexing only occurs infrequently, i.e. when the device is manufactured or repaired. FIG. 9 illustrates several examples of the use of flex PCBs in flex circuitry including flex PCB 112 with numerous ICs and passive components mounted on top of the PCB as shown in the inset 111. Another example of a flex circuit integrates mounted components 114 including a microcontroller and a humidity sensor as well as using the PCB conductive traces as an antenna 113.
Flex PCBs operating as flex circuits typically comprise two to six layers and contain components mounted on one or possibly both sides of the flex PCB. As described, flex PCBs are limited to “rarely-flexed” applications because of the mismatch between the flexible PCB and the rigid components mounted on it. The problematic use of flex circuits, i.e. flex PCBs with mounted components in applications with repeated flexing cycles, damage and breakage occurs because the components themselves do not bend even though the PCB does. Examples of component mounting failures are shown in FIG. 10A where LEDs mounted on a PCB 115 include electrical solder joints 116 connecting the LEDs to the PCB's traces. Cross sectional microphotograph 120Z illustrating copper lead frame 121 attached to the PCB by solder 123, clearly reveals that subjected to repeated bending and deformation, solder cracks 122Z results.
As shown in FIG. 10B, depending on the degree of the bending stress and the frequency of the flexing cycles, the magnitude of the cracks varies widely. For example in contrast to cross-section 120A where solder 123 exhibits no cracking, cross-section 120B exhibits crack 122B damaging around 20% of the solder's attachment width to leadframe 121. By subjecting the PCB to larger stresses or additional flexing cycles, the size of the crack will grow larger. For example, crack 122C in cross section 120C represents damage to over 33% of the solder joint, crack 122D in cross section 120D represents roughly 50% crack damage, and crack 122E in cross section 120E represents a crack 70% of the length of the solder contact to the PCB. In the extreme case shown in cross section 120F, crack 122F extends completely across the solder contact, the lead of leadframe 121 completely separates the lead from the PCB causing an electrically open circuit.
Cracking can also occur on solder joints mounting passive components such as resistors and capacitors. For example in FIG. 10C, cross-section 125 illustrates after repeated stressing passive component 126 attached to PCB by solder 123 exhibits solder cracking 122X. In extreme case shown in cross section 130, flex PCB 132 and conductive trace 133 resulted in cracking 134 of plastic package 131. Other potential defects from repeated flexing includes cracking 138 of bent lead 137 of gull-wing leaded package 138 shown in cross section 135 and solder ball cracking 144 of solder ball 143 connecting BGA or chip-scale package 141 to PCB trace 142 of PCB 146 shown in pictorially in cross section 140 and schematically in FIG. 10D.
The combination of rigid and flex PCBs further exacerbates the problem by requiring connections between the two. Such connections are subject to the same socket-plug failures as ribbon cables described previously.
Rigid-Flexible PCBs
Another variant of a flexible PCB, a rigid-flex PCB is a hybrid of flexible and rigid PCBs laminated into a single PCB with the flexible portion providing an interconnect between large rigid PCBs. Examples of a rigid-flex PCBs are illustrated in FIG. 11A and FIG. 11B. As shown, an intervening flex PCB connects one rigid PCB to another. Examples include a notebook motherboard with the flex PCB acting as an interconnection across the notebook's hinged display module.
As used today, the main advantage of a rigid-flex PCB is it eliminates the need for plugs and sockets to facilitate electrical connections between the rigid PCBs. Each flex PCB is merged into the rigid PCB, in a manner the same as any multiple layer PCB. Interconnection to the flex PCB is accomplished using multilayer via connections shorting rigid PCB layers to flex PCB layers as desired. The main disadvantage is due to the mismatch in mechanical properties between the rigid and flex layers, it is easy to rip the flex PCB by any force applied perpendicular to the plane created by the PCBs near the bar shaped interconnection area, i.e. in the z-direction as illustrated in drawing 170 of FIG. 12A where rigid PCB 171 connects to flex PCB 173 along a thin bar shared intersection expanded in cross section 173. Any substantial force in the z-direction may cause tearing of flex PCB 173 near the rigid PCB.
This unique rigid-flex PCB failure mode is illustrated in the schematic drawing and photo of a torn flex PCB in FIG. 12B. As shown, flex PCB 183 connecting rigid PCB 181 to rigid PCB 182 failed after repeated flexing resulting in flex PCB tear 184 adjacent to rigid PCB 181.
Multi-PCB System Failure
The use of rigid, flex, and rigid-flex PCBs or combinations thereof in multi-PCB electronic systems enables electronics to conform to any arbitrary shape, greatly expanding the application range of electronics. By 3D folding for example, PCBs can be squeezed into enclosures otherwise too small to accommodate required PCB surface areas. By conforming to curves surfaces, PCBs can be fit in motor casings, watch enclosures, miniaturized surveillance cameras, and more. By adjusting to better fit the contours of the human body, wearable electronics for sports applications as well as monitors and therapeutic devices for medical applications can benefit from increased sensor accuracy and improved treatment efficacy.
From an electronics system perspective however, such distributed circuits, i.e. ones where pieces of the circuit are implemented on different PCBs, suffer from numerous system reliability risks associated with communication among the various components. For example, FIG. 13A illustrates distributed electronic system 189A realized across three rigid PCBs 190A, 190B, and 190C and connected by flex PCBs 191A and 191B comprising connections 192 for power 193A, ground 193C and either analog or digital signals 193B as illustrated by the drawing inset expanding the magnification of connections 192 As shown each rigid PCB contains a different circuit or unique function in the overall system. For example, PCB 190A integrates circuit number 1, PCB 190B integrates circuit 2, and circuit 3 is integrated on PCB 190C. Circuit 1, 2 and 3 represent different functions without which the system will malfunction degrading performance or resulting in catastrophic system failure. The failure risk is exacerbated by the required interconnections, in the example shown as flex PCBs 191A and 191B which in a distributed system or in wearable electronics may represent large dimensions relative to the size of the PCBs being interconnected. In such distributed systems, tear 193 to flex PCB 191B may not just sever rigid PCB 190C from the rest of the system but likely can cause the entire system to malfunction or the software to crash. Such distributed systems are sensitive to single point failures and offer little or no protection from mechanical damage to the interconnections between its multiple rigid PCBs.
For example, in distributed electronic system 189B shown in FIG. 13B, tear 194B in the flex PCB results in an open circuit in the conductor carrying power 193A causing a temporary or permanent interruption in power leading to a total system failure. By contrast, in distributed electronic system 189C also in FIG. 13B, tear 194C in the flex PCB results in an open circuit in one or more conductors carrying control signals 193B resulting in system malfunction, affecting normal operation and depending on the function of the interrupted signals, possibly resulting in a total system failure.
Moisture & Corrosion Failures
Another physical mechanism that may result in immediate or gradual system malfunction is moisture-induced electrical failure. In the event that a PCB is immersed in or subjected to any conductive or slightly conductive fluid, an electrical short may result, either impairing or potentially damaging a circuit or system. Common examples of fluids include beverages, fresh water, and salt water. For example, in the photos of FIG. 14A, water damage results in localized defects 197C, 197D and 197E shorting out circuitry and impairing or disabling system operation. In wearable electronics, circuitry and PCBs may also be subjected to rain and to body sweat. Sweat is especially problematic because it contains salt and other electrolytes making it more electrically conductive. Continuous exposure to salty or acidic water can deposit salts on top of a PCB or result in corrosion of the PCB surface as shown in damage to the PCB surface 197B and to electrical leads and solder joints 197A. Failures may comprise electrical shorts or because of corrosion may also result in electrical open circuits. Operation of electrical systems in the presence of fluids, moisture, or high humidity may also result in the growth of conductive filaments as shown in photo 197G in FIG. 14B, or damage to PCB edge connectors as shown by 197F.
Coating flex PCBs with a protective layer is problematic because the coating invariably cracks with repeated flexing. Coating rigid PCBs is beneficial but does not support bendable or wearable PCB applications.
Conclusion
What is needed is a technology able to reliably interconnect a variety of printed circuit boards over a large area bendable to fit any shape, contour or form factor without being sensitive to moisture-related or mechanically induced interconnect failures. Such a system should be applicable to large area distributed systems, to ultra-compact systems, and to medical and wearable electronics designed to fit snuggly against anyone's body or conform to any shape, fixed or adapting to movement without breakage or electrical failure. Ideally, even in the event some breakage does occur, the system would still be able to survive the damage and continue operation even after being broken.